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The 82-year-old actress – who starred alongside Ross as Peggy Mitchell on EastEnders – was first diagnosed with the condition in 2014.
Since then, her condition has progressively deteriorated.
Opening up on his documentary Ross Kemp: Living With Dementia, Ross, 55, recalled the emotional moment he and his on-screen mum reunited.
He said: "[She said:] 'WHO are you? What are you doing here?'
"I've been in Barbara Windsor's house for just a few minutes. I'm sitting down with a cup of tea and a biscuit with my friend of three decades. I'm taken aback, but I can't show it.
"When I arrived, she greeted me with a hug, as one of her closest friends."
The Grant Mitchell actor continued: "Barbara was my TV mum on EastEnders and is hilariously funny and vivacious, with a rapier wit – we've always had a giggle, on and off set.
She clearly has no idea who I am.
"But now she clearly has no idea who I am."
Barbara lives with her husband and primary carer Scott Mitchell, 56.
Ross added: "On the wall opposite where I’m sitting on the sofa, Barbara’s husband Scott has put up a 'memory board' to remind her she is married to him, he loves her, and that this is her house – but mainly to remind her who she is.
"The impact of her dementia can lull you into a false sense of security."
Ross said the motivation for the ITV series was to gain an understanding of how families cope with the heartbreaking disease.
Meanwhile, his own grandmother was also diagnosed with dementia.
Earlier this year, Barbara's longtime friend Christopher Biggins revealed how the soap legend still "laughs a lot" despite her advancing Alzheimer's.
He told Best magazine: "I had lunch with Barbara recently. She's fantastic. We laugh a lot, she's great on the past too.
"She can remember all her Carry On jokes and also songs from the old time music hall. The other day, she sang 'The Boy I Love Is Up In The Gallery'.
"[Her memory] is not so good when she tries to remember things from day to day."
'Wonderful sense of humour'
Barbara's husband Scott also said that she's still got a sense of humour.
He revealed: "The thing that we know about dementia and Alzheimer's is that it's a progressive illness, so you know you're heading in one direction.
"So, yes, things progress but she still has that wonderful sense of humour, she can still laugh and have a giggle and watch telly, and we go out occasionally."
- Ross Kemp: Living With Dementia begins on ITV tonight at 7.30pm, and continues with three further episodes.
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